Composite structures are known for example of the type described in Italian patent application number 12441-A/90 entitled: "Method for the Production of Sandwich Structures" filed on 26 Mar. 1990 by the same Applicants, including a so-called three-dimensional textile which is formed from two sheets of textile placed face to face and connected together by a plurality of threads extending from the adjacent surfaces of the sheets.
In order to form such structures, the three-dimensional textile, impregnated with resin, is placed in a die and an expanded resin is injected into the space between the sheets and, in expanding, forces apart the sheets which then come into contact with the surfaces of the die. In such composite structures, the expanded resin determines the mechanical characteristics and the thermally insulating properties of the structure itself.
Study has also been made of the manufacture of a composite structure of the type described above which is at least partially hollow, this being made by impregnating a three-dimensional textile with a fluid thermo-setting resin and, instead of expanded resin, blowing in compressed gas in order to force the sheets apart and form an interspace between the sheets themselves.
These methods although having the distinctiveness of generating structures with sculptured geometries, in the construction of flat structures (panels) sometimes give geometric and dimensional irregularities (for example, undulations in the surfaces and a non-constant thickness).